(a) Technical Field
The present disclosure relates to a system and method for computing a motor speed and a rotor position using a hall sensor. More particularly, it relates to a method for computing a motor speed and a rotor position using a hall sensor, to enhance precision for computing the speed of the motor rotating at a high speed and a rotor position of the motor using the hall sensor.
(b) Background Art
Environmentally-friendly vehicles such as a fuel cell vehicle, an electric vehicle, a hybrid vehicle, and a plug-in electric vehicle include a plurality of motors for generating power and driving a vehicle and a hall sensor for detecting revolutions per minute (RPM) of each motor to more precisely control driving of the motors.
The hall sensors are mounted at predetermined angle intervals at a predetermined position of each motor and each hall sensor may be configured to generate on or off digital signals based on a rotation of a rotor of the motor to output position information of the rotor and perform a series of motor driving controls (e.g., computation of a motor speed, and the like) based on the position information. In particular, the typical method for computing a motor speed using a hall sensor will be described with reference to the accompanying FIGS. 1 and 2. A motor controller is configured to determine a time until the signal of the hall sensor is changed from high to low or from low to high, at a predetermined time interval, which is referred to as Tc.
Referring to FIG. 1, for example, when the change in the signal of the hall sensor is sensed at only m-th, the time when the signal is sensed becomes mTc and the motor speed is computed by the following Equation 1.
                                        ⁢                                          ⁢                      (                          Motor              ⁢                                                          ⁢              Speed                        )                          =                              1                          N              ×              m              ×                              T                c                                              ⁡                      [                          rev              /              s                        ]                                              Equation        ⁢                                  ⁢        1            
In the above Equation 1, m represents a period of change in the signal of the hall sensor, that is, the time until the signal of the hall sensor is changed from high to low or from low to high, Tc represents the predetermined time interval for confirming the signal of the hall sensor, and N represents the number of poles of the motor.
However, the typical method for computing a motor speed using a hall sensor has the following problems.
First, errors may occur during the calculation of the motor speed and the rotor position. As illustrated in FIG. 1, an error of ΔT1−ΔT2 occurs between a time Treal when a sensing value of the hall sensor is actually changed and a measured time mTc as in the following Equation 2, such that errors occur in the computation of the motor speed.Treal=mTc+ΔT1−ΔT2 Treal−mTc=ΔT1−ΔT2   Equation 2
Further, a position value of the rotor is confirmed based on a timing when the sensing value of the hall sensor is changed from low to high as in timing {circle around (1)} of FIG. 1. However, at the timing {circle around (1)}, there occurs a time error called ΔT1 between the time when the sensing value of the hall sensor is actually changed and the time when the controller senses the change in the sensing value of the hall sensor and similarly, a time error called ΔT2 occurs even at a time {circle around (2)} of FIG. 1, such that errors occur during the calculation of the rotor position.
Second, the following problems may occur due to the occurrence of errors during the calculation of the motor speed and the rotor position.
1) The computation errors of the motor speed may cause errors of a measurement value of a feedback speed used in a speed controller among configurations of the motor controller illustrated in FIG. 2, and thus an output current command value of the speed controller may fluctuate.
2) The computation errors of the rotor position may cause errors of the position value of the rotor used in a current controller, and therefore a ripple may occur in a current supplied to a 3-phase motor, such that the speed control may be unstable.
3) The instability of motor speed and current control may occur and control efficiency of the motor and the inverter may be degraded, due to the occurrence of errors during the computation of the motor speed and the rotor position, and heat generation of the motor may be promoted due to the degradation of the motor efficiency, and thus cooling performance may also be degraded.
Third, as the time interval Tc confirming the signal of the hall sensor decreases, the computation errors of the motor speed and the rotor position may be reduced, but a computation load of a CPU of the motor controller may be increased.
Generally, since the motor controller uses a switching frequency from several kHz to tens of kHz, the motor controller is configured to perform a complex computation involved in the motor control for a time of about 100 μs to determine a pulse width modulation (PWM) duty, and therefore the computation load is substantial. Therefore, it may be impossible to make the signal confirming period of the hall sensor infinitely short due to restrictions of the computation load of the motor controller.
Fourth, considering that the rotation speed of the motor used in a compressor for air supply of the fuel cell vehicle is increased as specifications of air supply pressure are increased, such that the motor is driven from tens of hundreds of RPM to hundreds of thousands of RPM, the occurrence of errors during the calculation of the motor speed and the rotor position using the hall sensor may cause the degradation in fuel efficiency and power performance of a fuel cell vehicle.
Meanwhile, to more accurately compute the motor speed and the rotor position, instead of the hall sensor, a resolver sensor may be used, but the resolver sensor is more expensive than the hall sensor and has a disadvantage in terms of a package for mounting the resolver sensor and thus is rarely used for other components other than a driving motor of the fuel cell vehicle.
The above information disclosed in this section is merely for enhancement of understanding of the background of the invention and therefore it may contain information that does not form the prior art that is already known in this country to a person of ordinary skill in the art.